1. Field of the Invention
This invention relates to molecular weight controllers useful in the polymerization of lactams to polyamides, which find particular application in the melt-spinning of filamentary structures.
2. Description of the Prior Art
Fibers comprised of polyamides, especially poly(epsiloncaprolactam), otherwise known as nylon 6, find widespread commercial use in view of their highly desirable physical, chemical, and aesthetic qualities.
The term "fiber" as used herein includes fibers of extreme or indefinite length (i.e., filaments) and fibers of short length (i.e., staple). The term "yarn" as used herein means a continuous strand of fibers.
Fibers of polyamides, including those of nylon 6, are usually prepared by the melt extrusion of molten polyamide through spinnerette plates containing small-dimensioned orifices. A common procedure for producing the polyamides and, in particular, nylon 6, involves the polymerization of epsiloncaprolactam at sufficient temperatures and in the presence of water which serves to initiate the polymerization. Polymers prepared in this manner, however, usually contain more than 10 percent water extractable material which usually comprises caprolactam monomers and linear and cyclic oligomers. The oligomers are mainly low molecular weight cyclic condensation products of epsiloncaprolactam. The presence of such water extractable material has an adverse effect on the performance of the polymer in the extrusion shaping of fibers and even larger-dimensioned articles such as pipes. To produce fibers having satisfactory physical properties, the water extractable content, which consists of oligomers and monomers (epsilon-caprolactam), should be less than 2.5 percent by weight of the polymer. The monomer content should be less than 0.5 percent by weight of the polymer.
Various procedures have been utilized to remove water extractable materials from polyamides prior to their melt spinning. The most common approach involves the hot water leaching of the nylon 6 in granular form to remove water extractable material, followed by thorough drying prior to its melt extrusion. Another technique employed has been to subject molten nylon 6, generally in the form of thin films or shallow layers, to an extremely high vacuum, which removes the relatively volatile epsiloncaprolactam monomer but has only a minor effect on the less volatile oligomer materials. Such purification techniques involve additional processing steps with an attendant increased manufacturing cost. In addition, regardless of the purification technique applied to the nylon 6, once the reaction mass is no longer under the influence of vacuum, the monomer can reform at high temperature within the polymer prior to spinning.
In order to reduce the extra manufacturing costs involving purifying polyamides, i.e., poly(epsiloncaprolactam), various procedures have been proposed for the continuous production of caprolactam having a reduced amount of water extractable material. Such procedures include, for example, the addition to the polymerization reaction of an organic acid material, or an organic basic material or combinations thereof as molecular weight controllers or chain terminators.
As described in U.S. Pat. No. 4,574,054, which is hereby incorporated by reference, in order to lower the formation rate of water extractable materials, it is necessary to synthesize polyamides having a low content of terminal amino groups. However, polymerization kinetics require a certain amount of terminal groups (both amino and carboxylic) and terminal amino groups are necessary in order to provide the polyamides with an affinity for anionic dyestuffs.
The present invention provides a molecular weight controller for polymerizing lactams to polyamides which overcomes or at least mitigates the above-described problems.